Projective displays use images focused onto a diffuser to present an image to a user. The projection may be done from the same side of the diffuser as the user, as in the case of cinema projectors, or from the opposite side. Prior art projection systems of the retro-reflective screen type used in, for example, movie theaters, as depicted in FIG. 1, typically produce a two-dimensional image by using a single projector 10 to project a single image at a time onto a screen 12. Screen 12 would have a linear light distribution such that all viewers of the image reflected by screen 12 would see the same two-dimensional image regardless of the positions at which they are located.
In many contemporary projection systems, the image can be generated on one or more “displays,” such as a miniature liquid crystal display device that reflects or transmits light in a pattern formed by its constituent switchable pixels. Such liquid crystal displays are generally fabricated with microelectronics processing techniques such that each grid region, or “pixel,” in the display is a region whose reflective or transmissive properties can be controlled by an electrical signal. In an liquid crystal display, light incident on a particular pixel is either reflected, partially reflected, or blocked by the pixel, depending on the signal applied to that pixel. In some cases, liquid crystal displays are transmissive devices where the transmission through any pixel can be varied in steps (gray levels) over a range extending from a state where light is substantially blocked to the state in which incident light is substantially transmitted.
When a uniform beam of light is reflected from (or transmitted through) a liquid crystal display, the beam gains a spatial intensity profile that depends on the transmission state of the pixels. An image is formed at the liquid crystal display by electronically adjusting the transmission (or gray level) of the pixels to correspond to a desired image. This image can be imaged onto a diffusing screen for direct viewing or alternatively it can be imaged onto some intermediate image surface from which it can be magnified by an eyepiece to give a virtual image.
The three-dimensional display of images, which has long been the goal of electronic imaging systems, has many potential applications in modern society. For example, training of professionals, from pilots to physicians, now frequently relies upon the visualization of three-dimensional images. Understandably, three-dimensional imaging also has numerous potential applications in entertainment. In many applications of three-dimensional imaging it is important that multiple aspects of an image be able to be viewed so that, for example, during simulations of examination of human or mechanical parts, a viewer can have a continuous three-dimensional view of those parts from multiple angles and viewpoints without having to change data or switch images.
Thus, real-time, three-dimensional image displays have long been of interest in a variety of technical applications. Heretofore, several techniques have been known in the prior art to be used to produce three-dimensional and/or volumetric images. These techniques vary in terms of complexity and quality of results, and include computer graphics which simulate three-dimensional images on a two-dimensional display by appealing only to psychological depth cues; stereoscopic displays which are designed to make the viewer mentally fuse two retinal images (one each for the left and right eyes) into one image giving the perception of depth; holographic images which reconstruct the actual wavefront structure reflected from an object; and volumetric displays which create three-dimensional images having real physical height, depth, and width by activating actual light sources of various depths within the volume of the display.
Basically, three-dimensional imaging techniques can be divided into two categories: those that create a true three-dimensional image; and those that create an illusion of seeing a three-dimensional image. The first category includes holographic displays, varifocal synthesis, spinning screens and light emitting diode (“LED”) panels. The second category includes both computer graphics, which appeal to psychological depth cues, and stereoscopic imaging based on the mental fusing of two (left and right) retinal images. Stereoscopic imaging displays can be sub-divided into systems that require the use of special glasses, (e.g., head mounted displays and polarized filter glasses) and systems based on auto-stereoscopic technology that do not require the use of special glasses.
Holographic imaging technologies, while being superior to traditional stereoscopic-based technologies in that a true three-dimensional image is provided by recreating the actual wavefront of light reflecting off a the three-dimensional object, are more complex than other three-dimensional imaging technologies. Thus, recent work in the field of real time thee dimensional electronic display systems has concentrated on the development of various stereoscopic viewing systems as they appear to be the most easily adapted commercially.
Recently, the auto-stereoscopic technique has been widely reported to be the most acceptable for real-time full-color three-dimensional displays. The principle of stereoscopy is based upon the simultaneous imaging of two different viewpoints, corresponding to the left and right eyes of a viewer, to produce a perception of depth to two-dimensional images. In stereoscopic imaging, an image is recorded using conventional photography of the object from different vantages that correspond, for example, to the distance between the eyes of the viewer.
Ordinarily, for the viewer to receive a spatial impression from viewing stereoscopic images of an object projected onto a screen, it has to be ensured that the left eye sees only the left image and the right eye only the right image. While this can be achieved with headgear or eyeglasses, auto-stereoscopic techniques have been developed in an attempt to abolish this limitation. Conventionally, however, auto-stereoscopy systems have typically required that the viewer's eyes be located at a particular position and distance from a view screen (commonly known as a “viewing zone”) to produce the stereoscopic effect.
One way of increasing the effective viewing zone for an auto-stereoscopic display is to create multiple simultaneous viewing zones. This approach, however, imposes increasingly large bandwidth requirements on image processing equipment. Furthermore, much research has been focused on eliminating the restriction of viewing zones by tracking the eye/viewer positions in relation to the screen and electronically adjusting the emission characteristic of the imaging apparatus to maintain a stereo image. Thus, using fast, modern computers and motion sensors that continuously register the viewer's body and head movements as well as a corresponding image adaptation in the computer, a spatial impression of the environment and the objects (virtual reality) can be generated using stereoscopic projection. As the images become more complex, this prior art embodying this approach has proven less and less successful.
Because of the nature of stereoscopic vision parallax can be observed only from discrete positions in limited viewing zones in prior art auto-stereoscopy systems. For example, any stereoscopic pair in standard auto-stereoscopy systems gives the correct perspective when viewed from one position only. Thus, auto-stereoscopic display systems must be able to sense the position of the observer and regenerate the stereo-paired images with different perspectives as the observer moves. This is a difficult task that has not been mastered in the prior art.
In light of the current state of the art of image projection, it would be desirable to have a system that is capable of projecting numerous aspects or “multi-aspect” images such that the user can see many aspects and views of a particular object when desired. It would further be useful for such viewing to take place in a flexible way so that the viewer is not constrained in terms of the location of the viewer's head when seeing the stereo image. Additionally, it would be desirable for such a system to be able to provide superior three-dimensional image quality while being operable without the need for special headgear. Thus, there remains a need in the art for improved methods and apparatuses that enable the projection of high-quality three-dimensional images to multiple viewing locations without the need for specialized headgear.